The temporal characterization of time-varying signals is a problem occurring often in science and technology. Instruments and apparatus capable of sampling analog signals are called waveform analyzers, transient recorders, digital oscilloscopes, data loggers, etc. The typical operation of such an instrument consists of the time-sequential conversion of the analog signal into a train of digital values which are then stored for later retrieval and analysis. This operation is limited by the speed with which analog signals can be sampled and converted into the corresponding digital representation.
To increase the sampling speed to very high values measured in nanoseconds and below, the analog signal is sampled at very high speed, and the acquired samples are represented by electrical charge packets that are stored in a suitable charge storage element, such as a charge coupled device (CCD). From this CCD, the charge packets can then be read out and converted into a train of corresponding digital values at reduced speed, requiring only limited analog-to-digital conversion rates.
An example of such a fast-in/slow-out (FISO) sampling device is described in U.S. Pat. No. 4,725,748 (R. Hayes et al., “High-speed data acquisition using multiple charge transfer delay lines”). It employs a linear CCD structure through which the signal charge packets are clocked at high speed. To both sides of this CCD line, two-dimensional CCD storage sections are placed which are tapping the high-speed CCD line at various spatial positions, and thus sampling the signal charge packets at various times. The two-dimensional CCD sections allow for intermediate storage of the signal charge packets and for reading them out through a common output node at reduced speed. This device requires clocking of the complete CCD line and charge signal sampling at the full sampling frequency, requiring complex, high-performance electronic driving circuitry.
The demands on this driving circuitry can be reduced with a suitable charge signal demultiplexer, requiring less complex and slower clocking circuitry, as taught in European Patent No. 0540105A2 (J. H. Noordeloos et al., “Sampling device for sampling analog signals and digital measuring instrument provided with such a sampling device”). Similar two-dimensional CCD storage and FISO readout structures with a common single output node are employed as described in U.S. Pat. No. 4,725,748. Although the complexity of the clocking circuitry is reduced, it is still necessary to provide a clock with the maximum sampling frequency to the demultiplexing structure, and the two-dimensional CCD charge storage and FISO structures require a substantial amount of space on the sampling device.
To simplify charge transport in the demultiplexing structure, a combined transport method using electrical drift in clocked charge coupled delay lines was invented in U.S. Pat. No. 5,528,643 (J. Hynecek, “Charge coupled device/charge super sweep image system and method for making”). This method allows the fast transport of charge packets in semiconductors, increasing the readout speed of CCD image sensors. However, the method only works for single charge packets, and it is therefore unsuitable to sample analog time signals where the creation and detection of complete trains of charge packets are required.
A simpler method for moving electrical charge packets through semiconductor material was proposed by K. Hoffmann, in Solid State Electronics, Vol. 20, pp. 177-181 (1977). It allows the fast and almost lossless transport of charge packets at a speed that can be controlled with a voltage, without the need of any clock signals. This is achieved with an MOS (metal-oxide-semiconductor) transmission line, consisting of an elongated layer of highly resistive material on top of an insulator covering a semiconductor. A voltage difference is applied to the two ends of the highly resistive layer, creating a spatially varying potential distribution at the interface between semiconductor and insulator. Charges packets that are introduced through a transistor into the semiconductor transmission line feel the spatially varying surface potential, and they move along the electric field lines to the region with lowest potential energy, at the other end of the transmission line. As a consequence, this device allows the fast and almost lossless transport of charge carriers along the length of the semiconductor device. However, since this transmission line is intended only as a charge transport device, it does not provide any means for the temporal sampling of the charge signals, other than the conventional charge detection circuits which could be placed at the end of the transmission line.
To overcome the limitations of these known methods and devices, the present invention describes a temporal sampling device for time-varying analog signals, allowing the sampling of one-time signals as well as the sampling and accumulation of recurring signals with very high temporal resolution below one nanosecond.